This invention relates to hydromechanical transmissions. More specifically, this invention relates to a method of shifting modes within a hydromechanical transmission.
Hydromechanical transmissions are frequently used in vehicles where shift quality is important and more specifically shift smoothness and continuous power delivery is important within the transmission. Within the hydromechanical transmission are a series of gears that make up separate modes within the transmission, usually a first mode for low speed and a second mode for high speed. These mode changes are usually made with a clutching element and are preferably made without a change in the transmission ratio by engaging the incoming clutch when the first and second modes are at near synchronous speeds.
For efficient space utilization, dog clutches have been used to accomplish this shift. A dog clutch may be used if the clutch element speed differential is low enough and the clutch torque direction and magnitude is satisfactory. On occasions, a clutch element synchronizer can be used to assist in achieving the satisfactory speed differential. Unfortunately, synchronizers have limited maximum torque capacity and also require torque to drop below a threshold to complete the shift.
Presently, in hydromechanical transmissions, during the shift there is a period of time when a first clutch element disengages from the first mode and the second clutch element has yet to engage the second mode. During this time, the speed of the transmission elements will drift in a direction of reducing torque. Consequently, when a shift is made with one of the hydrostatic units at or near zero displacement, there is not active control of this unit's speed. Therefore, the hydrostatic unit speed can either increase or decrease depending on whether the unit was acting as a pump or a motor prior to the initial shift. Conversely, there is little effect on the transmission function when a zero displacement unit changes speed in order to accomplish shifts. Consequently, when using dog clutches, a shift method must be used which accounts for the location of the hydraulic units.
Though advances have been made in the hydromechanical transmission art, problems still remain. When one of the hydraulic units is near zero displacement, the other is near zero speed. Thus, for the low speed unit it takes a relatively large change in displacement to produce a corresponding change in flow. Consequently, both low displacement and speed can cause tolerance and gain issues that interfere with achieving the desired speed and torque conditions necessary for a proper shift. Consequently there is a need in the art to develop a shifting method that will improve the shifting quality in a hydromechanical transmission in the presence of these tolerance and gain issues.
Therefore, it is a principal object of the present invention to achieve higher quality shifts within a hydromechanical transmission.
Another object of the present invention is to provide for a method of shifting that occurs in or near a torque dead zone of a hydromechanical transmission.
Yet another object of the present invention is to utilize dog clutches to improve the shifting process.
Another object of the present invention is to provide an active control of the hydrostatic units during the shifting process.
Yet another object of the present invention is to utilize an electronic controller that measures a plurality of variables in order to improve shifting quality.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.